The present invention relates to a wood molding process and, more particularly, to a wood molding process which imparts a stable fixed shape to molded woods.
Wood molding is a common practice for production of furniture, toys, sporting goods, musical instruments, commodities, barrels, and crafts. Making a round timber into a square timber by compression molding is another common practice. These wood molding processes are accomplished in any of the following ways.
(1) Softening by microwave heating and subsequent molding, followed by several repetitions of slow cooling and rapid secondary heating with the molded configuration retained for relieving internal stress due to molding. For example, molding round timber of Japanese cedar into square timber involves steps of microwave heating a round timber at about 120.degree. C. for about 30 minutes, gradually compressing the softened round timber into a square timber by means of a press, and repeating several times slow cooling and rapid secondary heating while retaining the molded configuration. Without the slow cooling and rapid secondary heating, the molded square timber would restore its original shape after repeated moisture absorption and drying.
(2) Softening by microwave heating and subsequent molding, followed by high-pressure steam treatment with the molded configuration retained.
(3) Softening by impregnation with liquid ammonia and subsequent molding, followed by removal of ammonia. (U.S. Pat. No. 3,282,313)
(4) Heating under pressure in an ammonia gas atmosphere, thereby giving molded timber with a uniform high packing density, followed by removal of ammonia gas. (U.S. Pat. No. 3,646,687)
The foregoing processes have their respective disadvantages as follows.
(1) Difficulties in process control and productivity on account of the necessity of repeating post-molding cooling and heating while keeping the molded configuration.
(2) Necessity for high-temperature heat treatment (e.g., at 200.degree. C. for 1 minute or at 180.degree. C. for 8 minutes) with high-pressure steam after molding. With heat treatment at 140.degree. C. or below, the molded wood returns to its original shape when exposed to hot water. The resulting molded wood decreases in strength and hence needs impregnation with an adhesive such as isocyanate resin.
(3) Pollution of the working and surrounding environments with ammonia which vaporizes at the time of molding. Liquid ammonia impregnated into wood converts wood cellulose into plasticized ammonia cellulose and stays in wood until molding. Instability in the molded shape because of reversion of cellulose III resulting from treatment with liquid ammonia to more stable cellulose I on prolonged heating in the presence of water.
(4) Poor productivity. Molding wood in hot ammonia gas at 120.degree. C. under a pressure of 0.1-10 kg/cm.sup.2 takes a long time, for example, 50-200 hours for birch.